Optical sensing device for fluid sensing and methods therefor

ABSTRACT

An optical spectral sensing device for determining at least one property of a fluid. The device has an elongated porous body, a first end and a second end, a solid-state optical emitter at the first end of the body oriented to emit radiation toward the second end of the body, and a solid-state optical detector at the second end of the body oriented to detect radiation emitted by the optical emitter. A package for detecting properties of a fluid includes a body defining a cavity, with a movable and biased carrier for an optical detector or emitter mounted in the cavity for increased reliability. A system for determining relative concentrations of fluids in a sample includes emitter/detector pairs operating at reference wavelength and wavelengths corresponding to absorption peaks of at least two fluids, and a processor for determining concentration based on measured data and calibration data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 U.S.C. §371 ofInternational Application No. PCT/US2012/041431, filed Jun. 7, 2012, andclaims priority from U.S. Provisional Patent Application Ser. No.61/520,308 entitled “LOW-COST IN-LINE AND IN-TANK FLUID QUALITY SENSORSBASED ON SPECTRAL MEASUREMENT PRINCIPLES” filed on Jun. 7, 2011, thesubject matter thereof being incorporated herein by reference, in itsentirety.

FIELD OF THE INVENTION

The present disclosure relates to optical sensors and associatedsystems. More particularly, it relates to optical sensors and fluidmonitoring systems used in, for example, the heavy equipment, automotiveand transportation industries.

BACKGROUND

The role of optical spectral measurements for the monitoring of staticand dynamic fluid systems is well established in the field ofspectroscopy. Traditional systems may include the use of a spectrometricmeasurement system, such as a spectrometer or photometer, opticallyinterfaced to a fluid stream, such as a liquid or gas. In the case ofspectrometer systems, commercial dispersive near-infrared (NIR) orFourier transform infrared (FTIR, near- and mid-IR) instruments oftenutilize various optical sensors used in transmission, transflectance (acombination of transmittance and reflectance) and internal reflectancemodes of operation. U.S. Pat. No. 7,339,657, hereby incorporated byreference in its entirety, discusses each of these modes of operation asimplemented into various optical sensor packages.

More generally, optical spectroscopy, for example, in the form ofinfrared spectroscopy is a recognized technique for the analysis andcharacterization of various types of fluids used in industrial,automotive and transportation applications, including lubricants,functional fluids and diesel emission fluids (DEF), which is markedunder the ADBLUE® trademark of Verband der Automobilindustrie E.V.(VDA). Such spectroscopic measurements can provide meaningful data aboutthe condition of the fluid and the fluid system during service. The terminfrared spectroscopy is used in the broadest sense, and includes bothnear infrared and mid-infrared, and covers the region from 700nanometers (nm) to 25,000 nm.

Infrared spectroscopy, as defined above, can provide measurement offluid quality, such as DEF quality, and fluid properties, by way ofexample only, oxidation, coolant contamination, fuel dilution, soot, andcontent. In most cases, this information is derived directly as ameasure of the chemical functionality, as defined by the characteristicvibrational group frequencies observed in the near infrared and infraredspectra. Further, the UV and visible spectra may provide informationderived from color and/or information derived from electronictransitions and can be applied to provide information about oxidation,moisture and additive content, by way of example.

While the infrared spectral region is definitive in terms of themeasurement of materials as chemical entities, the measurements can bedifficult to implement in terms of the materials used. Morespecifically, the optics and associated materials used in thesemeasuring devices are relatively expensive and do not always lendthemselves to easy replication for mass production.

Moreover, when multiple devices are implemented into a larger monitoringsystem used in, for example, automotive monitoring applications, thesesystems often become prohibitively large, complex, and expensive.Another factor to consider is the operating environment. If a monitoringsystem is to be used in a relatively benign environment, such as in alaboratory under standard ambient conditions or in a climate conditionedindoor facility, then a device construction of the prior art may beused. However, if there is a requirement to measure a fluid system in aless conducive environment, such as on a process line (indoors oroutdoors), on a vehicle, or a mobile or fixed piece of equipment, thenit is necessary to consider a system more capable of operating undersuch conditions. This may include considering the temperaturesensitivity of the components, as well as their robustness in terms oflong-term exposure to continuous vibrations. Additional factors forconsideration include size, thermal stability, vibration immunity andcost.

Alternative fluid measurement systems and techniques for fluid sensingand monitoring that address one or more of these considerations aredesired.

SUMMARY

According to one embodiment of the present disclosure, an opticalspectral sensing device for determining properties of a sample isprovided. The device includes an elongated porous body having a firstend and a second end. A solid-state emitter source is arranged at thefirst end of the body and a solid-state detector is arranged at thesecond end of the body. An electronics package is operatively connectedto the device for providing energy to the solid-state emitter, and forreceiving a signal generated by the detector. The body is configured tobe at least partially submerged in the sample, and the electronicspackage is configured to determine a value of the depth of the fluid orof the depth of submersion of the body, and to output at least one valueindicative of the depth of submersion of the body or depth of fluid.

A low-temperature safe sensor package is also provided. The packageincludes a housing defining an internal cavity therein for communicatingwith a fluid to be sampled. A sensor carrier is moveably arranged withinthe internal cavity and is biased into an operating position within thecavity by a spring element arranged between the sensor carrier and aportion of the housing.

A method for determining in a sample a concentration of a first fluid ina second fluid is also provided. The method comprises the steps ofdetecting a first intensity of radiation transmitted through the sampleby a first beam having a first path length at a reference frequency(f_(ref)); detecting a second intensity of radiation transmitted throughthe sample by a second beam having the first path length at a frequencycorresponding to an absorption peak of the first fluid; detecting athird intensity of radiation transmitted through the sample by a thirdbeam having a second path length at the reference frequency; anddetecting a fourth intensity of radiation transmitted through the sampleby a fourth beam having the second path length at a frequencycorresponding to an absorption peak of the second fluid. The temperatureof the sample is then determined. A value equal to (the secondintensity/the first intensity)−(the fourth intensity/the thirdintensity) is then calculated. Finally, a value of the concentration ofthe first fluid and the second fluid is calculated based on the value of(the second intensity/the first intensity)−(the fourth intensity/thethird intensity), the detected temperature, and stored calibration data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are schematic diagrams illustrating an exemplary fluidmonitoring system as implemented into a vehicle or heavy equipmentapplication.

FIGS. 2A and 2B are schematic diagrams illustrating exemplary methods ofintegrating monitoring systems according to embodiments of the presentdisclosure into a vehicle or heavy equipment application.

FIGS. 3A-3C are cross-sectional views of various optical packages whichmay be used by sensors according to embodiments of the presentdisclosure.

FIGS. 4A and 4B are cross-sectional views of insertion style sensorsaccording to embodiments of the present disclosure.

FIGS. 5A-5C illustrate embodiments of in-line (FIGS. 5A and 5B) andsubmersible (FIG. 5C) sensors according to embodiments of the presentdisclosure.

FIGS. 6A-6C illustrate an exemplary in-tank sensor for measuring bothfluid level and composition according to an embodiment of the presentdisclosure.

FIGS. 7A-7C are partial, exploded, and assembled perspective views,respectively, of an exemplary sensor and housing configured to protectagainst hard frost conditions.

FIGS. 8A-8D are graphical representations of the absorption spectra ofwater and DEF, analytical wavelengths for water and urea, exemplarylight-emitting diode (LED) wavelengths, and a resulting calibrationfunction derived therefrom.

FIG. 9 is a graphical representation of an exemplary LED emissionspectra and a DEF absorption spectra at 20 Celsius (C).

FIG. 10 shows a graphical representation of both temperature compensatedand uncompensated measurements with various DEF concentrations.

FIG. 11 is a graphical representation of an exemplary sensor output usedto measure DEF concentrations.

FIG. 12 is a graphical representation of the spectral response ofEthanol-Gasoline blends.

FIG. 13 is a graphical representation of the spectral response ofBio-Diesel-Diesel blends.

FIG. 14 is a process diagram illustrating a method of determining thedepth of a fluid according to the embodiment of FIGS. 6A-6C.

FIG. 15 is a schematic diagram illustrating a system for performingtemperature compensated fluid measurements according to an embodiment ofthe present invention.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, many other elements found in fluid measuringsystems, including those utilizing spectroscopy. However, because suchelements are well known in the art, and because they do not facilitate abetter understanding of the present invention, a discussion of suchelements is not provided herein. The disclosure herein is directed toall such variations and modifications known to those skilled in the art.

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. It is to beunderstood that the various embodiments of the invention, althoughdifferent, are not necessarily mutually exclusive. Furthermore, aparticular feature, structure, or characteristic described herein inconnection with one embodiment may be implemented within otherembodiments without departing from the scope of the invention. Inaddition, it is to be understood that the location or arrangement ofindividual elements within each disclosed embodiment may be modifiedwithout departing from the scope of the invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims, appropriately interpreted, along with the full range ofequivalents to which the claims are entitled. In the drawings, likenumerals refer to the same or similar functionality throughout severalviews.

The term “processor” when used herein generally refers to a circuitarrangement that may be contained on one or more silicon chips, and/orintegrated circuit (IC) boards, and that contains at least one CentralProcessing Unit (CPU), and may contain multiple CPU's. The CPU maygenerally include an arithmetic logic unit (ALU), which performsarithmetic and logical operations, and a control unit, which extractsinstructions from memory and decodes and executes them, calling on theALU when necessary.

Processors may take the form of a microprocessor, and may be a low powerCMOS processor with an embedded analog to digital converter, by way ofnon-limiting example only. The present invention is operable withcomputer storage products or computer readable media that containprogram code for performing the various computer-implemented operations.The non-transitory computer-readable medium is any data storage devicethat can store data which can thereafter be read or accessed by acomputer system component such as a microprocessor. The media andprogram code may be those specially designed and constructed for thepurposes of the present invention, or they may be of the kind well knownto those of ordinary skill in the computer software arts. Examples ofcomputer-readable media include, but are not limited to magnetic mediasuch as hard disks, floppy disks, and magnetic tape; optical media suchas CD-ROM disks; magneto-optical media; and specially configuredhardware devices such as application-specific integrated circuits(ASICs), programmable logic devices (PLDs), and ROM and RAM devices.Examples of program code include both machine code, as produced, forexample, by a compiler, or files containing higher-level code that maybe executed using an interpreter.

The term “electronics package” as used herein is to be understoodbroadly and includes any configuration of electronic components for usein providing power to components, such as LED's and detectors, controlsignals to such components, receiving data from such components,performing calculations and signal processing on data received from suchcomponents, storing received and processed data, and providing outputsof such data to monitoring and display systems. Such packages mayinclude discrete analog and digital components, batteries, integratedcircuits configured to include multiple analog and/or digital logiccomponents, general purpose and special purpose processors, data storagedevices of all descriptions including magnetic, capacitive, randomaccess, read-only and other non-transitory storage media, wireless andwired transmitters, receivers, and transceivers, and other devices, indiscrete and integrated form.

The detectors and emitters of all embodiments disclosed herein may beintegrated into and integrally formed with electronic packages, such ason printed circuit boards such as control boards of such packages.Alternatively, the detectors and emitters may be configured to bemounted separately from control boards and other electronic devices

Fluid measuring/monitoring systems according to embodiments of thepresent disclosure take into account factors of size, thermal stability,vibration immunity and cost, and are configured to facilitate massproduction. Sensors and monitoring systems according to embodiments ofthe present disclosure may simplify the complex arrangements of theprior art by providing a wavelength specific light or energy source (orsources), a device for interfacing with the sample, and one or moredetectors. These simplified spectrometric/photometric systems can bemade relatively small and compact compared to the large and expensivemonitoring systems of the prior art, while retaining their functionalityand reliability in harsh environments.

These systems may include the use of solid-state light emitters (e.g.LEDs), low-cost, solid state detectors, integrated with opto-electronicsthat reduce temperature dependency effects, low-cost optics that may bemass-produced such as by molding techniques (if required), and low-costpackaging. Residual temperature effects may be handled by thermalmodeling and the application of compensation algorithms.

The sensor devices described in this disclosure may be implemented asmonitoring devices for water-based fluids, such as DEF and coolants, inaddition to fuels, lubricants and other functional fluids used inautomotive vehicles, heavy equipment, and various forms oftransportation that involve dynamic fluid lubricant and power conversionsystems. They may include sensor devices for monitoring engine oils,transmission oils, hydraulic oils and fluids, turbine oils, coolants andany fluid system that protects mechanical moving parts or transmitspower to moving parts. Throughout the disclosure, the term fluid isconsidered in the broadest sense, and can include gases and vapors,which include off-gassing vapors from fuels, slip and bypass gases fromcombustion zones, and exhaust gases. In one or more configurations, thesensor can be operated immersed in the fluid, and measurements can bemade in a static environment such as a tank or storage vessel, or in amoving environment, such as a fuel line or exhaust pipe. It isunderstood that the period of measurement may vary from less than asecond, to a few seconds, to periods of days or longer, such as forsystems where the change in fluid composition (chemistry) changesslowly, if at all. When used for fluid quality assessment the sensor isintended to monitor for changes in composition, including contaminationfrom the use of an incorrect fluid.

Referring generally to FIGS. 1A-1C an exemplary fluid monitoring systemis shown as implemented into an automotive or heavy equipmentapplication. As set forth above, sensors according to embodiments of thepresent disclosure may be suitable for fluid monitoring in all aspectsof equipment operation. With reference to FIGS. 1A and 1C, forapplications such as DEF quality monitoring, a sensor 10 may be locatedwithin a given fluid stream, such as in the feed lines or in a fluiddosing system 2. Further, a sensor may be configured as a submersiblecomponent located within a feed tank 1 (e.g. a DEF or fuel tank).

Referring generally to FIGS. 1B and 1C, sensors according to embodimentsof the disclosure may also be used for oil condition monitoring (e.g.oxidation and nitration) in gasoline and natural fired engines. For thisapplication, sensing devices may be located at the output side of anengine's 3 primary (or secondary) filtration system, where a filter 8 isinserted into the stream on the return side of the filter-housing block.Advantages of mounting the sensor on the filter block include convenientaccess, external mounting, and reduced operating temperature.Alternative positions for the sensors described herein may include thetransmission 4, the coolant system 5 and the rear axle 7. Another sensorposition is within a relatively cooler location of the exhaust system 6,wherein a heat-insulated probe and sensor can monitor exhaust gas forspecies such as NOx (see also FIG. 4B). While many of the embodiments ofthe present disclosure are described in the context of sensor devicesinstalled on a vehicle, or a combustion engine powered system, thisserves only as appropriate examples. The devices are, as indicated,intended for use in all forms of fluid measurement systems.

With reference to FIGS. 2A and 2B, with one or more sensors on board avehicle or piece of equipment, the measured data may be provided to adisplay or on-board data handling system. Embodiments of the presentdisclosure may communicate sensory output back to the operator/drivervia one or more alarms, alerts, displays or status lights. Referring toFIG. 2A, in one implementation, a standalone system 20 includes afunctional display and associated interface hardware 14 directlyresponsive to the output of a sensor 10 for communicating data to anoperator. This type of interface may be advantageously implemented as aretrofit to an existing vehicle or piece of equipment. With reference toFIG. 2B, in other embodiments, however, the measurement systems may bemore fully incorporated into the vehicle's original equipment (OE)control/computer systems. For example, the output of one or more sensors10 may be provided to the vehicle's management system, including anon-board computer or data management processor 9. From this managementsystem, sensory output data may be provided to, for example, an operatordisplay 11, an external communication device 12 (e.g. a transmitter forcommunicating with a remote monitoring system), or stored into memoryvia a data bus 13 for further processing or retrieval. It should beappreciated that sensors 10 may receive power provided by data bus 13 orthrough the normal power distribution system of the vehicle.

Sensors according to embodiments of the present invention generallycomprise low-power consumption devices internally operating at 3.5 to 5volts, and configured to receive and process input voltages normallyfound on vehicles and ranging from 12 to 40 volts DC. The sensors can beconfigured with various electronics packages, such as a simple digitaloutput device or a smart sensor that provides processed numerical data.The output from the sensor can be provided directly to any suitable typeof display, such as a simple status light, for example, a three stateLED: green (OK), yellow (warning) and red (alert or problem), or to analpha-numeric or a graphical display, for example, an LCD display.Alternatively, the sensor can provide a standardized format output (e.g.SAE J1939) to the vehicle or equipment data bus 13, such as the CAN bus(e.g. a 5V-High-speed-CAN, 250 kbit, IS011898) of a vehicle, supplyingdiagnostic data (OBDI/II) either to an on-board computer, which in turnsupports and intelligent sensor output display 11.

Sensors described herein may utilize any suitable optical package,operating in a variety of modes (e.g. internal reflectance ortransflectance formats), as set forth in U.S. Pat. No. 7,339,657. Forexample, with reference to FIG. 3A, sensors according to embodiments ofthe present invention may comprise a light source 32, a reflector 33 anddetector 34 configured for an internal reflectance mode of operation.Likewise, transmittance mode (FIG. 3B) and light scattering 24 (FIG. 3C)modes may be implemented. These embodiments may include the formation ofan open path or channel 31 in reflector 33 to allow fluid to flowbetween a pair of opposing optical surfaces, thus providing atransmission path for the optical beam. In the transmittance mode, theabsorption measurement is proportional to the thickness of the channel31. Accordingly, this channel may be formed as, for example, a narrowslot for opaque or highly absorbing fluids, or as a wider cavity orchannel for lower absorption samples.

It should be noted that in the transmission mode (FIG. 3B), multiple LEDsource components may be configured in close proximity or co-packagedwith a near-common light path. The system can utilize a comparable setof detectors (or a reduced detector set) dependent on the final beampath and divergence through the optical structure (not illustrated).This is an important facet and is utilized in the fluid quality monitorsfor enabling multi-component monitoring. For example, a DEF qualitymonitoring system may be provided that utilizes LEDs (and correspondingdetectors) at a reference wavelength, and wavelengths corresponding toabsorption peaks of two or more fluids in a two or more fluid sample,such as three LEDs at wavelengths of 810 nm (reference), 970 nm (whichcorresponds to absorption peaks in water and hydroxyl), and 1050 nm(which corresponds to an absorption peak in urea).

Referring generally to FIGS. 4A and 4B, embodiments of the presentdisclosure include insertion-style sensors used for measuring one ormore properties of liquids (sensor 40, FIG. 4A) and gases/vapors (sensor41, FIG. 4B). Sensors 40,41 are shown utilizing an internal reflectanceconfiguration. Each sensor 40,41 comprises an electronics package 49 forcontrolling the sensor's opto-electronics including LED(s) 42 anddetector(s) 44 in combination with a reflector 43 (e.g. a slottedreflector). Note that the number of detectors used is a function of theillumination area (from the source), the desired sensitivity (as definedby the signal-to-noise ratio, SNR) and the available space. In theillustrated embodiment, sensor 40 may be used in a liquid insertionenvironment configured for longer path lengths. In this layout reflector43 is located at a distance that provides the desired net path length,while taking into account the total path traversed by theretro-reflected radiation. Sensor 41 comprises an extended path length45 and is intended for gas and vapor measurements. Other embodiments mayinclude an optional protective heat barrier 46 intended for exhaust gasor other high-temperature applications (e.g. through exhaust pipe 48).The introduction of the gases into sensor 41 is passive and relies onpermeation of the gases/vapors through a medium such as a stainlesssteel gauze or membrane 47.

Referring generally to FIGS. 5A-5C, sensors according to embodiments ofthe present disclosure also include in-line and submersible packages.For example, FIG. 5A is a cross-sectional view illustrating an in-line(flow-through) sensor 50 with an adjustable retro-reflective insert 53,and an electronics package 51 including at least one light source 52 andat least one detector 54. This interchangeable insert 53 may be used forfine adjustment of the optical path length, or reflector type, withoutthe need to replace the entire sensor package. As illustrated, energyfrom light source 52, typically a multi-wavelength device, passesthrough the fluid in chamber 55 and back to detector 54 along the pathshown in FIG. 5A. The transmitted energy (typically UV-visible-NIR)interacts with the sample fluid, with the characteristic absorptions ofthe fluid modifying the light transmission of the fluid, and issubsequently sensed by detector 54. The selectivity of the absorption isdefined by the wavelengths of the source(s), which may also be effectedby the combination with the optical filters integrated with thedetectors. FIG. 5B shows an exemplary perspective view of sensor 50 ofFIG. 5A, including connectors 57,58 in communication with chamber 55 forinterfacing with a fluid feed path (e.g. a fuel line).

In an alternative embodiment illustrated in FIG. 5C, sensor 51 operatesin a “staring” mode, wherein electronics package 51 includes a lightsource 52 (e.g. LED) and a detector 54 located generally opposite oneanother, between which is arranged a sensing area 59. While sensor 51may be intended to be used as a submersible sensor that can be locatedwithin, for example, a fluid dosing tank, this configuration is notconstrained to in-tank usage, and can be integrated into a flow-throughsystem.

The geometries shown for the sensors illustrated in the previous FIGS.4A and 5A-5C are based on the absorption profiles for the common fluidsin the NIR to mid-IR spectral regions providing path lengths from a fewmillimeters to about 50 mm. The gas measurement path length for the gasimplementation shown in FIG. 4B can be longer, and with the foldedgeometry a path length of up to 500 mm can be considered. However, analternative in-tank embodiment of the present disclosure for measuringboth fluid level and fluid quality/composition in an in-tank applicationis illustrated in FIGS. 6A-6C. In this staring mode embodiment, the pathlength is defined by the volume of fluid in a tank 68 and theanticipated depth of fluid. In this embodiment, the depth to be measuredmay be of a liquid, and not of a gas, the volume above the level of theliquid being of gases. More specifically, sensor 60 is housed in anelongated porous body or housing 61, which may be in the form of ahollow tube, which may be cylindrical. The active components (e.g. alight source/LED(s)) 62 of the sensor may be mounted at the lower end ofhousing 61 (FIG. 6C). The receiver or detector 64 may be arranged on anopposite end (FIG. 6B), for example, at the top of tank 68 with theassociated control and data processing electronics 65. The light source62 and detector 64 may be so arranged and oriented that the light source62 transmits radiation along body from one end to another, and mayfurther in an embodiment transmit radiation through the hollow interiorof the body 61 to detector 64. The mounting of the light source 62within the hollow body may tend to protect light source 62 againstphysical shock. Wiring to transmit power and control signals to lightsource 62 may be within the hollow body 61; in embodiments, light source62 may be packaged in a sealed unit including one or more batteries orother internal source(s) of energy. In this embodiment, the absoluteabsorption measured may be correlated initially to path length or depthof fluid, and the relative absorptions of the fluid components aredetermined and correlated to the ratio(s) of the main components. Morespecifically, referring generally to FIG. 14, sensor 60 of FIGS. 6A-6Cmay be used in a measuring process 90 for determining the depth of avolume of fluid. In one embodiment, a light source (e.g. LED 62) isoperative to transmit light energy through a volume of fluid (step 92).This energy is subsequently detected in step 94 a detector (e.g.detector 64). In step 96, a comparison is made between the magnitude ofenergy transmitted by LED 62 and the magnitude of the energy received bydetector 64 to calculate energy absorbed by the fluid. Finally, in step98, this absorption value is compared to a predetermined absorption vs.depth relationship, which may be stored in a memory device incorporatedin electronics 65, for estimating or determining the depth of the fluid.This estimated or determined depth value may be output to, for example,a display device in step 99. As will be understood by one of ordinaryskill in the art, these calculations may be performed by processingcomponents incorporated into the control electronics (e.g. electronics65).

Each of the above-described embodiments of sensors that form the basisof a solid-state measurement system utilize a spectrally selectivesource (e.g. an LED) and a detector. Embodiments described herein mayutilize optical interfacing based on, for example, direct line of sightcoupling of the source(s) and detector(s) (i.e. staring mode), or by atransflectance configuration, as set forth in greater detail in U.S.Pat. No. 7,339,657.

In one embodiment, the detectors comprise one or more silicon-baseddetectors. Silicon photodiode detectors have the advantages of highsensitivity over a broad spectral region (nominally 350 nm to 1100 nm),linearity, robustness, availability of a large number of packagingoptions, and low cost. Other solid-state light detectors may beimplemented without departing from the scope of the present disclosure,such as InGaAs, PbS/PbSe and MEMS components.

Regarding the light sources, LEDs offer the advantages of color orwavelength specificity, constant output, low power consumption, nosignificant thermal output, output frequency modulation ability,compactness and robustness, availability in a large number of packagingoptions, and extremely low cost. A relatively wide range of spectralwavelengths are commercially available off the shelf for LED sourcesfrom 240 nm (far UV) to 3000 nm (mid-IR). Longer wavelengths are alsoavailable. Likewise solid-state detectors can be combined with opticalfilters to provide for wavelength selection. This integration can be aphysical combination of a filter element with the detector device, orthe filter may be processed onto the detector device at the wafer level.

Moreover, certain LEDs are commercially available with matchingdetectors, examples being the short-wave NIR LEDs, which are commonlyused for remote “infrared” monitoring and control. Certain LEDs canoperate at two or more states producing more than one wavelength (suchas red, yellow and green) from a single device. This enables a verycompact design using a single source and single detector, and where theoutput for individual wavelengths is differentiated by differentmodulation frequencies. In certain measurement systems up to four orfive or more unique wavelengths (for example: blue, green, yellow, redand NIR wavelengths) will be monitored, each as individual wavelengths,each detected by a single (or multiple) detector, and differentiatedbased on modulation frequency. The multiple channels will be modeled toprovide color profiling and multiple component determinations. LEDpackages including multiple LEDs may be used in the implementation ofthis configuration of the opto-electronics.

Embodiments of the present disclosure may also implement one or moreintegrated circuits for performing optical data processing, opticalcompensation, temperature compensation, analog and digital signalprocessing, and external communications. By way of non-limiting exampleonly, embodiments of the sensors described herein may be comprised ofone or more independently modulated LEDs coupled to an optical feedbacksystem for monitoring the outputs of the LEDs, independent of thesampling channel. This system may include compensation for drift in theoutput of the LEDs as a function of temperature. The feedback detectormay be located in close proximity to the system detector (e.g. after theoptical interface) to model the response changes of the detector system.In an alternative implementation of the LEDs, a reference wavelength isused as a baseline reference point. Such a reference is located at awavelength that does not absorb or interact with the fluid. An exampleimplementation is used for DEF measurements, wherein an 810 nm LED isused as a reference wavelength. This secondary wavelength provides areference independent of sample absorption, and as such can provide adirect ratio I₀/I which is used to calculate the effective absorption(proportional to species concentration): absorption=−log(I₀/I), where I₀is the reference intensity, and I is the intensity after the sampleabsorption. The optical and electronics system can be a singleintegrated circuit board or device, possibly featuring (but notexplicitly) application specific integrated circuits (ASICs) for thesignal handling, computations and data communications. This integratedopto-electronic component may be encapsulated, and may include someimaging optics, accomplished for example by some form of molded opticsin front of the source(s) and detector(s).

The placement of the opto-electronic elements (i.e. the LEDs anddetectors) is important to ensure optimum imaging through the opticalinterfacing structure. In a standard environment, with moderateoperating temperatures, the opto-electronics is close-coupled to theoptical interfacing structure. Typical distances are expected to be fromabout 1 mm to 1 meter (1 m) or more. At the shorter distances, noadditional imaging optics are contemplated. However, at longerdistances, supplemental lenses made from glass or plastic may be placedin front of the LED source(s) and detector(s) to improve image quality.Alternatives will include the use of light conduit, from the opticalinterfacing structure to the opto-electronics. Light conduits can be inthe form of glass or plastic rods (index matched or otherwise) oroptical fibers.

Packaging of the embodiments of the present disclosure may includefabricating housings from low-cost materials. Examples can includealuminum moldings or extrusions, machined plastics, plastic moldings andextrusions, and porous metallic mesh, as in the case of submersiblesensors (FIGS. 5A-6C). Fluids such as DEF may be aggressive to materialssuch as aluminum, and metals such as stainless steel. Components ofsensor packages may be made of plastics such as polyolefins,polysulfones and polyethers (such as DELRIN® brand acetal resin of E.I.DuPont de Nemours and Company), by way of example, to prevent corrosionor damage from the fluid. The selection of material will be based on therequirements of the application and cost. In cases where hightemperature applications are involved (80° C. or higher), a provisionfor providing external cooling fins and the use of thermally insulatingmaterials between the optical structure and the opto-electronics areprovided as options in the design.

As set forth above, sensor packages must be able to reliably perform inharsh environmental conditions. For example, fluid sensors in anautomotive application encounter temperatures from −40° C. to 80° C. forexternal installations, and −40° C. to 130° C. for under the hoodapplications (for engines) for fluid flow, and up to 200° C. forinstantaneous storage temperatures. Sensors have recommended operationaltemperature ranges for specific applications. A primary specification isthat the sensor can survive the temperature range without sustainingphysical, mechanical or electronic damage. A further temperaturespecification is a range for actual operation. This is typically tied tothe working temperature range of the fluid. A practical example is DEF,wherein the fluid freezes below −11° C., and can degrade at temperaturesabove 60° C.

Freezing of aqueous based systems is especially problematic where thereis a captive area wherein fluid exists or flows (e.g. in a closedsystem). For example, water can expand up to 10% in volume as itfreezes. Certain fluids, such as certain DEFs, also expand in volumeupon freezing. Thus, it is important to provide a method thataccommodates this expansion. Without consideration, the structureholding the fluid, or the sensors included therewith, can bemechanically stressed, and can break or fracture.

Referring generally to FIGS. 7A-7C, embodiments of the presentdisclosure incorporate a configuration of an optical sensor package,such as an in-line fluid property sensor, that provides hard frostprotection and prevents or mitigates physical damage upon the freezingof a fluid contained therein. These embodiments include a mechanism thatbehaves as a piston working against the counter pressure of a hightension spring. More specifically, an embodiment of a sensor 70 mayinclude a housing, such as a two-part housing including a lower portion74 having ports 75 (e.g. an inlet and an outlet) arranged thereon, andan upper part 72 (FIG. 7C). Lower portion 74 of the housing defines aninternal cavity 71 in communication with ports 75 for providing a fluidto be sampled thereto. A piston-like sensor carrier 76 is moveablyarranged within internal cavity 71. The carrier 76 may be elasticallymounted in cavity 71, such as by flexible biasing fingers joiningcarrier 76 to an inner wall of cavity 71, thereby both biasing andretaining carrier 76. A spring element 80, which may be a Bellevillewasher, is arranged between a portion of the housing and sensor carrier76. An optical package 73 is arranged on and carried by sensor carrier76. Sensor carrier 76 is sealed to an internal wall of lower portion 74by sealing element 79, which may be a double o-ring seal, renderingcavity 71 a closed cavity for fluid transmission via ports 75. As afluid within cavity 71 expands with freezing, if the frozen materialengages and applies force in excess to the holding force of the springelement 80 to optical package 73 and/or carrier 76, carrier 76 isdisplaced generally upwards against the biasing force of spring element80. This displacement relieves the stresses that might otherwise causebreakage or damage to either the housing or the optical package. As thefrozen fluid melts with increased temperature, optical package 73 isurged back to its operating position against a fixed mechanical stop 81defined in the housing via pressure exerted by the spring element. Thisspring bias and fixed stop 81 arrangement ensures that path length andoptical alignment integrity is retained, or re-realized, when carrier 76is returned to the operating position despite the movement of theoptical package during freezing conditions. The spring element 80 may beselected such that the carrier 76 is not displaced under normal fluidoperating pressures. A circuit board 83 may be provided, includingcontrol circuits, for operating the sensor's optical package.

When used in a flow mode application, below the freezing point, solidmaterial is generally retained within sensor packages. Even as thesesystems thaw with increased temperature, the short-term retention ofsolid material can nonetheless constrict the flow of fluid through thesensor. In order that the sensor not constrict the system, there may bea requirement to thaw the sensor, or to prevent the fluid containedtherein from freezing, to enable system operation. This may be providedvia an embedded heating element 78 arranged within sensor housing 72,74.Heating element 78 may comprise a physical element (e.g. an electricallyconductive polymer or other material having electrically resistiveproperties) provided, for example, around an exterior portion of cavity71, or may be integrated into sensor housing 72,74. In such anembodiment, circuit board 83 may further be configured with atemperature detector, and to provide electrical power to heating element78 in response to detecting a temperature at or below the freezing pointof the fluid. Circuit board 83 may further be configured to detectfrozen material via processing optical signals. In another embodiment,the heating component is arranged externally to an outer wall of thesensor package in the form of a heating blanket.

Referring generally to FIG. 8A, using embodiments of the sensorsdescribed herein in a simple binary fluid system, such as in a DEFsystem, the absorption spectrum of the two main components (i.e. water81 and DEF 82) may be separated spectrally. Measurements ofconcentration are obtained by the use of two or more LEDs that representthe analytical wavelengths for the analysis. The wavelengths of theseLEDs are indicated in FIGS. 8B and 8C, wherein analytical wavelengthsfor water, urea and a reference wavelength are defined. The measurementcan be enhanced by performing the measurement as a differential, wherethe water response is referenced against the DEF response. Using normalspectral absorption calculations based on the absorption of theanalytical wavelengths for water and urea referenced against a referencewavelength, a calibration function 85 for DEF can be derived (FIG. 8D).

Experiments performed over various operational temperature rangesindicate that both electronics and many fluids exhibit a temperaturesensitivity that result in inaccuracies in measured parameters.According to one embodiment herein, this hysteresis can be modeled byobserving the responses of the sensor with different temperaturesettings for the sensor immersed in various types of fluid. From theseobservations a series of response curves can be derived. Mathematicalfitting has shown that these functions are reproducible and are easilyfitted to a simple polynomial function. More specifically, both thefluid and the sensor temperature response functions can be representedby a simple 2^(nd) order polynomial. An exemplary calculation forperforming this thermal modeling is outlined herein.

In FIG. 15, an LED-based optical sensor 100 is schematically illustratedand configured to measure the DEF composition of a fluid. Exemplarysensor 100 may comprise two optical packages A,B, each comprising twoLEDs. Package A contains 810 nm and 970 nm wavelength LEDs 101,102,while package B contains 810 nm and 1050 nm wavelength LEDs 101,103. Inthe exemplary embodiment, there is an 810 nm LED 101 in each packagebecause the 810 nm light is not affected by water or DEF and, as such,they can be used as a reference and may be employed to compensate forvariations resulting from differences in path length. In this way, aratio of “1050/810” represents the relative amount of 1050 nm lightintensity compared to the 810 nm intensity. The same is true for an“970/810” ratio. Optical packages A,B further comprise respectivereference detectors 105 responsive to the output of reference emitters101, as well as detectors 106,107 responsive to emitters 102,103,respectively. Sensor 100 further comprises a processor 109 responsive toa temperature sensor 110 for measuring the temperature of a fluid 104 tobe sampled, and for performing the below-described steps for calculatingthe composition of the fluid. Memory device 108 is provided for storingpredetermined temperature to absorption/intensity ratios of DEF andwater. More specifically, memory device 108 may store calibration data,including values of intensity of radiation transmitted through water atthe reference frequency; the frequency corresponding to an absorptionpeak of water; the frequency corresponding to the absorption peak ofDEF; values of intensity of radiation transmitted through DEF at thereference frequency; the frequency corresponding to an absorption peakof DEF; and the frequency corresponding to the absorption peak of DEF.

In operation, sensor 100 turns on one LED at a time in sequence, thelight from the LED is transmitted through a specific volume of the DEFfluid. As the light traverses the fluid, certain chemical bonds in thefluid absorb energy at specific wavelengths of light. For example, FIG.9 illustrates the LED emission spectra and the DEF (also called DEFinterchangeably) absorption spectra at 20 C. As shown, the OH bond inwater absorbs at 970 nm, while the NH bond in urea absorbs at about 1050nm. By measuring the relative energy detected by the detector, theamount of absorbed light at these two wavelengths is measured, and byhaving a reference for each LED to normalize optical path variations,the concentration of urea in water can be calculated.

In practice, the nominal urea concentration in DEF is 32.5% as itprovides the lowest freezing point; this concentration of urea in wateris considered to be 100% DEF. Accordingly, an algorithm according to anembodiment of the present invention may use the 1050/970 absorptionratios of pure water to be 0% DEF and the 1050/970 absorption ratios ofpure DEF to be 100% as endpoints in calibration:

DA(T) represents the difference between the scaled 1050 nm and 970 nmLEDs at a given temperature in pure DEF:

$\begin{matrix}{{{DA}(T)} = {{1050\text{/}810{B({DEF})}} - {970\text{/}810{A({DEF})}}}} \\{= {{{da}\; 2*T\; 2} + {{da}\; 1*T} + {{da}\; 0}}}\end{matrix}$

DT(T) represents the difference between the scaled ratio of pure water,minus the scaled DEF ratio:

$\begin{matrix}{{{DT}(T)} = {{\left( {{1050\text{/}810B} - {970\text{/}810A}} \right)W} -}} \\{\left( {{1050\text{/}810B} - {970\text{/}810A}} \right){DEF}} \\{= {{{dt}\; 2*T^{2}} + {{dt}\; 1*T} + {{dt}\; 0}}}\end{matrix}$

A scaling term O(T) may be provided for normalizing the two 810 nmsignals if needed:O(T)=o2*T ² +o1(T)+02

The 1050/970 absorption ratio of the “measured” fluid of unknowncomposition is then linearly scaled using the 0% to 100% ratiospreviously calibrated:DM=100*O(T)*1050raw/810Braw−100*970raw/810Araw

Finally, the DEF composition of the fluid can be calculated according tothe following relationship:% DEF=100*((DT(T)+DA(T))−DM/DT(T)

As absorption ratios are a strong function of temperature at variousconcentrations, the 0% and 100% set points may be adjusted for theactual temperature during measurements.

The calculated DEF composition comprises a linear extrapolation,including a normalizing function for the 810 nm LED differences (O(T)),and is a function of the fluid temperature. FIG. 10 provides bothtemperature corrected and uncorrected outputs from an exemplary sensor,illustrating the functional benefits of the above-described temperaturecorrection algorithm.

In determining concentration by an electronics package according to anembodiment, calibration data including temperature-dependent linearinterpolation data between pure first fluid and pure second fluidintensity data may be stored in a memory device and accessed by aprocessor.

Referring generally to FIG. 11, exemplary outputs of a sensor forchanges in fluid concentration are shown. According to one embodiment ofthe present disclosure, using a control computer responsive to theoutput of one or more sensors, error codes can be generated that showthe concentration being below a predetermined acceptable level. In thecase of DEF this predetermined value 110 may be approximately 80%. Othererrors codes that can be considered based on relative and/or absoluteabsorption responses of the LEDs include empty sensor, dirty sensor,dirty fluid and/or wrong fluid, as it pertains to the presence ofcoolant or fuel, for example.

While the above-described embodiments of the present discloser have beendescribed primarily in the context of an aqueous-based system, it shouldbe understood that the applications of the sensors described herein arenot limited to this fluid, and mixtures such as coolant blends can beconsidered. Also, non-aqueous systems can be considered, such as fuels.For example, the determination of ethanol content of gasoline-ethanolblends may be especially useful given today's use of flex-fuelapplications. An example spectral response function for gasoline-ethanolsystems is illustrated in FIG. 12. Likewise, another fuel system thatcan be measured by the quality/composition sensor is the amount ofbio-diesel used in biodiesel blends. These blends involve mixtures withpetroleum diesel, where the biodiesel content may range from zero, B0,to 5% B5, to 100% B100. With the use of, for example, a 6-LED system,the sensor can be adapted to monitoring bio-diesel blends. A spectralresponse function for this system is illustrated in FIG. 13.

Example applications for the embodiments described herein include thefollowing:

-   -   a) Quality assessment and composition monitoring of diesel        emission fluid (aka DEF and AdBlue®),    -   b) Composition of blended fuels, including bio fuels, such as        biodiesel blends and gasoline-ethanol blends,    -   c) Monitoring of gases and vapors, with an example of NOx        components in exhaust and blow-by gases,    -   d) the monitoring of oxidation/acidity in transmission and other        lubricating oils,    -   e) the measurement of oil condition in gasoline and natural        gas-fired engines based on the formation of oxidation and        nitro-oxidation products,    -   f) the measurement of dispersed water (elevated levels) in        hydraulic and lubrication oil systems,    -   g) the measurement of turbidity, which can result from water,        air entrainment and/or particulates or other insoluble materials        in functional fluids,    -   h) the measurement of coolant condition, based on color,        composition and turbidity,    -   i) the measurement of marker materials for fluid compatibility,        usage and/or condition (color markers added to indicate chemical        changes), including fuel markers,    -   j) the monitoring of battery acid condition (acid strength),        based on a color indicator, etc.    -   k) rear axle fluid monitoring for level and breakdown    -   l) the measurement of fluid density based on refractive index

Exemplary embodiments of the present disclosure may include:

A urea quality sensor (UQS): A sensor based on optical transmissionmeasurements with a path length defined by the spectral method ofmeasurement. The fluid is considered to be a two-component system,involving water and urea as the designated and only ingredients, andwhere spectral measurement is based on the unique absorptions of theamino functionality of the urea, CO(NH₂)₂, and the hydroxylfunctionality of the water, H₂O or HO—H. Those of skill in the art ofoptical and vibrational spectroscopy will recognize that these arewell-defined and unique functionalities and can be measured in at leastfive regions of the total spectral range from the visible to themid-infrared. For convenience and in-line with the goal of defining acost effective solution the measurement region selected is the shortwave near infrared, where wavelengths are selected from 970 nm to 1050nm for the measurement of these two functionalities. These wavelengthsmay be monitored by LEDs with nominal outputs at 970 nm and 1050 nm. Asset forth above, the measurement can be maximized for dynamic range by adifferential measurement, referenced to an internal standard wavelengthat 810 nm to provide a calibration function. This can be done byconsideration that there is only two components that are supposed to bein the fluid.

Blend composition for bio-fuels: There are two common usages ofbio-fuels for automotive and combustion engine application and those arefor biodiesel (fatty acid methyl esters or FAME) and for ethanol. Inboth cases the fuel is used as a blend, with standard, hydrocarbondiesel for biodiesel, and with standard gasoline for ethanol. These aresometimes designated as B-blends (B0 to B100) for biodiesel and E-blends(E0 to E100) for ethanol blends, where the number designates thebio-fuel content. Like the previous application for DEF infraredspectral signatures can be defined that occurred repeatedly throughoutthe near infrared and mid-infrared, and that these can be convenientlymeasured in the short wave near infrared. The measurement for ethanol ingasoline is similar to that of DEF, where two analytical wavelengths,defining the hydrocarbon CH and the ethanol OH can be selected and usedrelative to a common reference at 810 nm (no absorptions at this point).The measurement of the biodiesel is a more complex and requires morewavelengths to be used. In this case CH information is used for bothcomponents, in one case the CH associated with the hydrocarbon is usedand in the other the CH from the carbon adjacent to the esterfunctionality is used.

Monitoring of gases and vapors: Optical spectroscopy can be used for thedetection and composition monitoring of gases and vapors, and thesemeasurements can be performed throughout the entire spectral region fromthe UV to the mid-infrared. At low concentration a longer path length isrequired and an example of the extension of the path is provided in theretro-reflective configuration of the sensor, as illustrated in FIG. 4B.There is a desire to have a low-cost measurement of NOx. This may beaccomplished in the UV by the use of UV LEDs, for monitoring NO and NOx.This measurement mirrors the method used industrially for the gas phasemeasurement of these two NOx emission components.

Monitoring density via refractive index: Refractive index varies asfunction of material composition and concentration. These correlate wellto density changes in a fluid. Both density and viscosity are used forfluid systems such as DEF for a bulk measurement of materialconcentration, with urea being the relevant component in DEF. Severalcommercial sensors utilize density or density related measurements tomonitor changes in urea concentration of DEF. The optical sensordescribed in U.S. Pat. No. 7,339,657 uses a refractive sensor tip thatcan be adapted to measure refractive index. Changes in the index aremonitored as changes in optical attenuation, and these can becorrelated, by calibration, to changes in urea content. This role forthe sensor is not limited to the measurement of DEF concentration, andthe sensor may be applied to other combinations of fluids, includingfuel blends and coolant mixtures.

Monitoring of oxidation and nitration products in gasoline and gas-firedengines: It has been satisfactorily been demonstrated that the opticalspectrum can model and trend both oxidation and nitro-oxidation ifmultiple wavelengths are monitored in the visible and short wave NIRregions. As the oil oxidizes and degrades, extended double bondstructures are formed as part of aldol condensations that take place inthe degradation pathway. These materials eventually become the insolubleorganic sludge that separates from the oil after extended use. As theextended double-bond structures form, the absorption wavelength of thesematerials shifts to the red end of the spectrum, and eventually into theshort-wave NIR. They may be tracked by monitoring the visible (green,yellow, red) and the NIR wavelengths. Also, the formation of nitrocomponents, from the NOx components may also be tracked in the visible.

Monitoring of oxidation and acid number in automatic transmissions: Thered dye used in Dexron automatic transmission fluids can be demonstratedto act as an acid-base indicator, reflecting the condition and theacidity of the fluid during use. The acid number of transmissions usedin buses is an issue relative to warranty claims. An on-board sensorcapable of modeling acid value based on the visible monitoring of thedye can provide an early warning to unacceptable acid numbers (relativeto warranty). A sensor configured in a similar manner to the oxidationsensor described can be adequate, but probably without the need for theNIR channel.

While the foregoing invention has been described with reference to theabove-described embodiment, various modifications and changes can bemade without departing from the spirit of the invention. Accordingly,all such modifications and changes are considered to be within the scopeof the appended claims. Accordingly, the specification and the drawingsare to be regarded in an illustrative rather than a restrictive sense.The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may beutilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations of variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

What is claimed is:
 1. A system for determining in a sample aconcentration of a first fluid in a second fluid, comprising: at leastone sensor for detecting: a first intensity of radiation transmittedthrough the sample by a first beam of a reference wavelength; a secondintensity of radiation transmitted through the sample by a second beamof a first wavelength; a third intensity of radiation transmittedthrough the sample by a third beam of the reference wavelength; a fourthintensity of radiation transmitted through the sample by a fourth beamof a second wavelength; a processor in communication with the at leastone sensor, the processor configured to calculate, based on the detectedfirst, second, third and fourth intensities, a value of theconcentration of the first fluid in the second fluid; and a memorydevice in communication with the processor for storing calibration data,wherein the processor is configured to calculate the value of theconcentration of the first fluid in the second fluid based on the valueof (the second intensity/the first intensity)−(the fourth intensity/thethird intensity), and the stored calibration data.
 2. The system ofclaim 1, further comprising a temperature sensor in communication withthe processor for detecting a temperature of the sample.
 3. The systemof claim 2, wherein the processor is further configured to calculate thevalue of the concentration of the first fluid in the second fluid basedon the value of the detected temperature.
 4. The system of claim 1,wherein the first and second beams comprise a first path length.
 5. Thesystem of claim 4, wherein the third and fourth beams comprise a secondpath length different from the first path length.
 6. The system of claim1, wherein the first wavelength corresponds to an absorption peak of thefirst fluid.
 7. The system of claim 6, wherein the second wavelengthcorresponds to an absorption peak of the second fluid.
 8. The system ofclaim 7, wherein the reference wavelength is selected such that thefirst and third beams are not absorbed by the fluid.
 9. The system ofclaim 8, wherein the reference wavelength is about 810 nm, thewavelength corresponding to an absorption peak of the first fluid is 970nm, and the wavelength corresponding to an absorption peak of the secondfluid is 1050 nm, whereby the system is configured to determine aconcentration of urea in water.
 10. The system of claim 1, wherein theat least one sensor comprises first, second, third and fourth sensorsfor detecting the first, second, third and fourth intensities,respectively.
 11. The system of claim 10, wherein each of the first,second, third and fourth sensors comprise a detector and emitter pair.12. A method for determining in a sample a concentration of a firstfluid in a second fluid, comprising: detecting a first intensity ofradiation transmitted through the sample by a first beam of a referencewavelength; detecting a second intensity of radiation transmittedthrough the sample by a second beam of a first wavelength; detecting athird intensity of radiation transmitted through the sample by a thirdbeam of the reference wavelength; detecting a fourth intensity ofradiation transmitted through the sample by a fourth beam of a secondwavelength; determining a value of the concentration of the first fluidin the second fluid based at least in part on the detected first,second, third and fourth intensities; and accessing stored calibrationdata, wherein the step of determining a value of the concentration ofthe first fluid in the second fluid is further based on the value of(the second intensity/the first intensity)−(the fourth intensity/thethird intensity), and the stored calibration data.
 13. The method ofclaim 12, further comprising the step of detecting a temperature of thesample.
 14. The method of claim 13, wherein the step of determining avalue of the concentration of the first fluid in the second fluid isfurther based on the value of the detected temperature.
 15. The methodof claim 14, wherein the calibration data comprises data indicative of:(a) values of intensity of radiation transmitted through the first fluidat (i) the reference wavelength; (ii) a wavelength corresponding to anabsorption peak of the first fluid; and (iii) a wavelength correspondingto an absorption peak of the second fluid; and (b) values of intensityof radiation transmitted through the second fluid at (i) the referencewavelength; (ii) the wavelength corresponding to the absorption peak ofthe first fluid; and (iii) the wavelength corresponding to theabsorption peak of the second fluid.
 16. A method for determining in asample a concentration of a first fluid in a second fluid, comprising:detecting a first intensity of radiation transmitted through the sampleby a first beam of a reference wavelength; detecting a second intensityof radiation transmitted through the sample by a second beam of a firstwavelength; detecting a third intensity of radiation transmitted throughthe sample by a third beam of the reference wavelength; detecting afourth intensity of radiation transmitted through the sample by a fourthbeam of a second wavelength; and determining a value of theconcentration of the first fluid in the second fluid based at least inpart on the detected first, second, third and fourth intensities,wherein the first and second beams comprise a first path length, andwherein the third and fourth beams comprise a second path length. 17.The method of claim 16, wherein the first wavelength corresponds to anabsorption peak of the first fluid, wherein the second wavelengthcorresponds to an absorption peak of the second fluid, and wherein thereference wavelength is selected such that the first and third beams arenot absorbed by the fluid.
 18. The method of claim 17, wherein thereference wavelength is about 810 nm, the wavelength corresponding to anabsorption peak of the first fluid is 970 nm, and the wavelengthcorresponding to an absorption peak of the second fluid is 1050 nm. 19.A method for determining in a sample a concentration of a first fluid ina second fluid, comprising: detecting a first intensity of radiationtransmitted through the sample by a first beam of a referencewavelength; detecting a second intensity of radiation transmittedthrough the sample by a second beam of a first wavelength; detecting athird intensity of radiation transmitted through the sample by a thirdbeam of the reference wavelength; detecting a fourth intensity ofradiation transmitted through the sample by a fourth beam of a secondwavelength; accessing stored calibration data; and determining a valueof the concentration of the first fluid in the second fluid based atleast in part on the detected first, second, third and fourthintensities, and the stored calibration data, wherein the calibrationdata comprises data indicative of: (a) values of intensity of radiationtransmitted through the first fluid at (i) the reference wavelength;(ii) a wavelength corresponding to an absorption peak of the firstfluid; and (iii) a wavelength corresponding to an absorption peak of thesecond fluid; (b) values of intensity of radiation transmitted throughthe second fluid at (i) the reference wavelength; (ii) the wavelengthcorresponding to the absorption peak of the first fluid; and (iii) thewavelength corresponding to the absorption peak of the second fluid; and(c) temperature-dependent interpolation data between pure first fluidand pure second fluid intensity data.
 20. The method of claim 19,wherein the calibration data further comprises temperature-dependentlinear interpolation data between pure first fluid and pure second fluidintensity data.